The long term goal of this proposed research is to understand the mechanisms of apoptosis in malignant and primary T cells. Fas is a member of the tumor necrosis factor receptor superfamily and is expressed on activated T cells and many malignant cells. Treatment with anti-Fas antibody leads to apoptosis in many but not all the tumor cells. A similar situation can also be found for T cells from different developmental stages. Immature T cells is particularly susceptible to Fas-induced apoptosis while mature T cells are quite resistant to the apoptotic-effect of anti-Fas antibodies. Mort 1 /FADD is a protein with a death domain, a protein-protein interaction domain that is also found in the cytoplasmic tail of Fas and tumor necrosis factor receptor p55. In sensitive cells, addition of anti-Fas antibodies triggers Mort1 to associate with Fas. Interference of this association process by a dominant negative Mort1 protein leads to inhibition of apoptosis. In several malignant cell lines that are resistant to Fas-mediated apoptosis, Mortl was found not to associate with Fas. Thus, Mortl-Fas association is a key step in Fas-induced apoptosis that might determine susceptibility of a cell to anti-Fas-mediated cell death. Lymphomas, thymomas and T cell hybridomas as well as transgenic and gene-targeted mutant mice will be used to understand the role of Mortl in apoptosis of malignant and primary T cells. The following specific aims are proposed. First, structure-function analysis of the Mortl protein will be performed using a Fas/Mortl chimeric protein. Second, the molecular process of Mortl/Fas association will be examined in malignant and primary T cells. Third, Mortl-deficient mice will be generated and analyzed. Fourth, the role of Mort I and one of its associating protein in apoptosis of immature T cells will be examined. The above aims should lead to a better understanding of the mechanisms of peripheral and central tolerance and why some tumor cells are resistant to Fas-induced apoptosis. The information obtained might lead to means by which apoptosis in resistant tumor cells might be induced and to halting of the onset of cancer.